Telecommunications articles and devices, such as cables and service wires, terminal blocks that enclose splices, junctions, other connections, and other electrical devices, are often exposed to damaging environmental conditions. Yet, they are required to maintain their mechanical, electrical, and environmental characteristics for a service life of at least 40 years. In the past, telecommunications service wires were strung above ground between poles, and, occasionally, placed under water. Increasingly, these wires are being buried beneath the ground for aesthetic reasons and to protect them from extremes of temperature, rain, snow, ice, high winds, and downed tree limbs and the like. However, even when placed underground, these articles are subject to mechanical shock during back filling, moisture, the possibility of immersion, and exposure to salt and other corrosive materials.
While typically housed in an enclosure, terminal blocks, containing splices, junctions, and other connections between wires or between wires and telecommunications devices, are often vulnerable if not properly sealed against environmental hazards, such as moisture and corrosive elements. Typically, silicone grease lubricant, an elastomer seal, or mastic sealing strips have been used in an attempt to seal out moisture. While these sealants offer some protection, moisture can often seep into the enclosure and cause a short circuit, which will interrupt telecommunications service, particularly when the terminal block is buried under ground.
Telecommunications service wires are normally well protected. Service wires normally comprise a bundle of individually insulated copper wires, protected by a metal shield, which covers the bundle of wires, and an outer sheath made from plastic or other insulating material. This construction is normally sufficient to protect the wires. However, if the protective outer sheath of a service wire is cut or otherwise broken, water can seep into the wire and run along its length. This is particularly true when an immersed wire is cut. To protect against this situation, the wire is filled with a water resistant filling composition.
At a splice, junction, or other connection, the metal shield and outer protective sheath of the service wire must be removed to connect wires from two or more service wires or devices. These connections are required to splice long service wires together, to connect service areas, and to make service connections for individual customers. In an unprotected device, corrosion of the contacts or short circuits caused by moisture or other outside elements will eventually interrupt service. Replacement or repair of buried telecommunications articles, as well as any interruption of service, can be particularly difficult and expensive. Therefore, an enclosure filled with a protective, moisture resistant filling composition, must be provided to protect telecommunications articles from moisture and corrosion.
A variety of telecommunications articles, which require protection against moisture, are illustrated in a brochure entitled "Electrical Products," published by 3M. In providing protection for telecommunications articles, care must be taken to ensure that the electrical, mechanical, and environmental qualities of the article are not impaired. Thus, in formulating a protective filling composition for use in telecommunications articles, the protective filling composition must not only provide protection from moisture and corrosion, but also must not impair the electrical, mechanical, and environmental qualities of the telecommunications article. Moreover, the protective filling composition must not chemically attack the components of the service wire or the enclosure of the terminal block.
The requirements of protective filling compositions for telecommunications articles are not easily satisfied. A protective filling composition must remain stable throughout the temperature range of use of the service wire and terminal block, which should extend at least from -40.degree. to +70.degree. C. If the maximum working temperature of the protective filling composition is not at least as high as that of the electrical components, the filling composition may be subjected to a temperature sufficient to reduce its viscosity to the point that it runs out of the enclosure, exposing the article to moisture and corrosion. In addition, protective filling compositions must be fluid enough over the temperature range experienced during installation for the enclosure to completely fill the enclosure with the composition without the risk of breaking or over-stressing the electrical components.
The protective filling compositions should also be cohesive enough to ensure that compression of the enclosure will not result in the formation of voids in either the interior or surface of the composition. Also, the protective filling composition should be soft enough to allow such compression without subjecting the electrical components, to any significant strain, but, yet, resist flow under gravity as well as the hydrostatic pressures likely to be applied to the article if the end of a service wire is exposed in a flooded manhole.
It is known in the art that gelling agents such as clays and hydrophobic fumed silica may be employed to provide gelled oil formulations for use as effective protective filling compositions. Hydrophobic fumed silica, however, is expensive since relatively large amounts of fumed silica must be used to provide useful gels. Use of hydrophobic fumed silicas, moreover, can cause the resulting gels to be abrasive.
A need therefore exists for cable grease protective filling compositions that have improved properties against moisture and corrosive materials, and which avoid the abrasiveness of silica based gel formulations.